Three-dimensional accelerometers can be attached to objects, and can measure the acceleration of the object in three dimensions. As part of these measurements, the accelerometer measures accelerations on the object caused by gravity.
Generally, a personal fall detection system comprises a user worn sensor module that is arranged to wirelessly send out an alarm signal to the remaining part of the system in the event that an emergency situation is detected. Such a sensor module may comprise an alarm button that the user can press to have the module send out the alarm signal or a detector that detects if a user has fallen, whereby the sensor module automatically sends out an alarm signal.
Many existing automatic fall detection solutions are based on measuring acceleration of the daily activities and falls of the user with a fall detector attached to the body, usually at the waist. The basic principle of these fall detection solutions is to detect the large or significant impact caused by the body hitting the ground during a fall. Some solutions can also detect the change in orientation of the body caused by a fall, which improves the success rate in detecting falls.
However, the existing solutions still have an unacceptably high false alarm rate. This is because measuring the change in body orientation in conjunction with detecting the occurrence of a large impact is often not sufficient to distinguish all daily activities of a user from a fall.
The fall detection system can be provided with additional functionality to generate more information about the activities of the user, which can improve the performance. In particular, adding a displacement measurement sensor to the sensor module can improve the performance. This is because going from a standing position to falling onto the ground results in the height of the sensor module decreasing by about the height from the waist to the ground, if the sensor module is attached to the waist. This displacement information is useful for distinguishing some daily activities from falls and improves the fall detection performance.
Integrating a barometer into an accelerometer-based fall detection system that allows absolute height to be measured from air pressure, fulfills the above requirement. However, including this type of additional component in the sensor module leads to increased energy consumption and size for the system. In addition, measurements by barometers can be adversely affected by changes in the environment, such as a change in the ambient pressure or temperature.
Therefore, there is a need for an alternative methodology for measuring the displacement of a sensor module that includes an accelerometer.